1. Field of the Invention
The present invention relates generally to a valve train for an internal combustion engine and more particularly to a mechanism for varying the timing of the valves of an internal combustion engine in order to obtain optimum efficiency and performance of the engine throughout the operating speed range.
2. Description of the Prior Art
Because of the fact that internal combustion engines for automotive vehicles must operate under widely varying speed and load conditions, the timing of the intake and exhaust valves of such engines is chosen so as to provide a resonable degree of efficiency and performance throughout the expected range of speeds and loads. Such timing, however, does not provide optimum efficiency and performance throughout any particularly range of operating speeds and loads. Accordingly, efforts have been made to improve the efficiency and performance of automotive internal combustion engines, particularly those employing poppet-type intake and exhaust valves, by varying the timing of such valves in relation to the working cycles of their respective cylinders.
One of the mechanisms having been heretofore developed for varying the timing of the valves of an internal combustion engine in accordance with varying operating conditions, as disclosed in the U.S. Pat. No. 3,413,965, utilizes an elongated lever pivoted at one end and extending parallel to a rocker arm located in the actuating train of a reciprocating valve, with the inner surface of the lever contacting the outer surface of the rocker arm. The contacting surfaces are contoured so that the lever serves as a fulcrum for the rocking motion of the rocker arm. During the rocking motion, the contact point between the lever and the rocker arm moves along the surfaces. An eccentric portion of a rotatable shaft contacts the outer surface of the lever and rotation of the shaft pivots the lever, thereby changing the crankshaft angle at which the valve begins its operational event.
While the above mechanism has accomplished its desired objective, it has not proved entirely satisfactory for the reason that a large torque is required for rotating the rotational shaft, resulting in a large loss of energy leading to deterioration in the efficiency and performance of an engine as well as a large-sized hydraulic driving system for the drive of the rotatable shaft. This is due to the fact that a valve spring, when the associated valve is unseated to open, strongly pushes the lever against its pivot shaft and the rotatable shaft, thus subjecting the lever and the rotational shaft to large frictional forces. When the mechanism is used in an engine having four cylinders or more, the rotatable shaft is always subject to large frictional force since in such an engine there is always at least one valve which is unseated to open.